[FWDLK] Radial tire harshness

[<LARSPAINTR@xxxxxxx>]

Well I hope for the last time,  I have FOUND the quintissential (whatever
that means) reason for radial tire harshness at in-town speeds, when putting
same on older cars.  The entire article is over 5 pages but an exerpt:

Car Life June 1967  article RADIALS

Why have the worlds carmakers been so slow to even experiment seriously with
radial tires?  The answer can be summed up in a few words--- slow speed ride
harshness.  Radial tires deliver a rough ride, especially at low speeds.
When a set of radials is put on a standard car (not originally designed for
radial tires), a harsh, thumpy ride that can be heard as well as felt, is the
result.  Its as if the stiff belt between the tread and carcass can be felt.
Small bumps, such as tar strips and railroad tracks, are especially
noticeable.  and the ride over such minor irregularities seems much worse at
low speeds up to 35 or 40 mph.  At highway speeds the tires smooth out and
provide, if anything, a better than bias ply ride.
  Bajer says its not primarily the stiff that causes ride harshness.  "What
happens is that the radial cords are acting something like a separate set of
spokes of the wheel," he explains. "when the belt runs over a bump it tries
to shift backward a little, or actually rotate around the tire bead.  The
radial ply 'spokes' resist this backward shift and give an actual
circumferential springing effect.  But every spring has its natural vibration
frequency.  In this case (of radials) its between 40-50 cps.  When the
frequency 0f road bumps synchronizes with this natural frequency of the
carcass, the ride harshness and thumpy sound is encountered.  This happens at
speeds up to 40 mph."
     "But at higher speeds, the radial tire tends to give a smoother ride
than the bias ply type.  This is because centrifigal force pulls outward on
the belt and tends to stiffen it up substanatially.  It literally glides
right up over the bumps.  Its as if you're riding on hoops that are connected
to the wheel by springs (the belts)- but without the synchronous problem
because youre above the natural frequency of the (pseudo) springs.  This is
better than bias plys becasue you're always working against rubbing friction
between the plies, and a continual tread and carcass distortion at the
contact patch".  END of quote from article

  I must add that the last sentence has to do with the inherent tread
distortion of bias plys at higher speed, there is an extreme distortion of
the contact patch AND the tire itself, causing vibration and 'squirrelyness'
of the entire tire.   Some of you readers may recall seeing test photos of a
bias ply tire running at 80 mph and the tread looks abysmally distorted.  The
radials will not get this distortion, thanks to the stabilizing design of the
plies.

Many of us have radials on our old wheels but the above is the most detailed
and factual coverage of WHY there is an decrease of low speed ride comfort if
we retrofit our cars.  Interestingly there is no commentary on rim widths.
And the impact problem is such that it will transmit into the old rims, that
will often allow a full wheel cover to be thrown off in spite of its
apparently adequate fit.  My 51 Merc lost 2 fine Lincoln turbine covers,
thanx to this secondary problem.  And I know of others that have similarly
lost rims.  No its not to back fit, its to to wheel (rim) flex which is
caused by the road irregularities transmitted into the older style rim, then
the slight distortion plus centrifigal force causing the wheel cover to
loosen and eventually get thrown off.

Modern design has long solved the low speed ride harshness problem due to
suspension tuning, again  some of us recall optional RTS (Radial Tuned
Suspensions) on some cars, mostly GM in the late 70's.  At that time both
bias belted and radial tires were available, and this option tuned out that
harshness. Since radials are universal now, all suspensions are designed for
them.  In fact so well designed we can have 50 and 55 profile tires on cars
and still have an extremely plush ride with the bonus of quick steering
response and mega traction.

All FYI

Lars

Lars